A Metathetic Approach to [5/5/6] Aza-Tricyclic Core of Dendrobine, Kopsanone, and Lycopalhine A Type of Alkaloids

Article abstract of DOI:10.1055/s-0039-1690620

A concise synthetic approach to [5/5/6] tricyclic pyrrolidine core of dendrobine is reported. This methodology relies on the construction of β-hydroxylactams by NaBH ₄ -I ₂ reduction followed by reaction of allylsilane with the aid of Lewis acid to generate alkenyl lactams in good yields. Further, ring-opening metathesis (ROM) followed by ring-closing metathesis (RCM) were used to assemble the [5/5/6] aza-tricyclic skeleton of dendrobine. This short synthetic route has been expanded to assemble tricyclic [5/5/8] system with pentenylboronic acid.

Full text of DOI:10.1055/s-0039-1690620

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2019, 51, A–H
paper
A
en
S. Kotha, S. Pulletikurti
Paper
Synthesis
A Metathetic Approach to [5/5/6] Aza-Tricyclic Core of Dendrobine,
Kopsanone, and Lycopalhine A Type of Alkaloids
Sambasivarao Kotha*
O
Sunil Pulletikurti
N
O
O
H
H
Me
Department of Chemistry, Indian Institute of Technology
Bombay, Mumbai 400076, India
srk@chem.iitb.ac.in
H
H
O
N
N
NAr
NAr
H
sunil.p@iitb.ac.in
H
H
kopsanone
H
O
O
dendrobine
OH
H
H
• 4 Steps (reduction, allylation, ROM-RCM)
• >50% overall yield
• diastereoselective
N
N
H
O
lycopalhine A
Received: 18.07.2019
Accepted after revision: 07.08.2019
Published online: 13.09.2019
O
O
O
O
N
OH
H
H
DOI: 10.1055/s-0039-1690620; Art ID: ss-2019-z0253-op
H
Me
H
H
Me
O
H
N
N
N
N
Abstract A concise synthetic approach to [5/5/6] tricyclic pyrrolidine
core of dendrobine is reported. This methodology relies on the con-
struction of -hydroxylactams by NaBH4-I2 reduction followed by reac-
tion of allylsilane with the aid of Lewis acid to generate alkenyl lactams
in good yields. Further, ring-opening metathesis (ROM) followed by
ring-closing metathesis (RCM) were used to assemble the [5/5/6] aza-
tricyclic skeleton of dendrobine. This short synthetic route has been ex-
panded to assemble tricyclic [5/5/8] system with pentenylboronic acid.
N
H
O
H
MeOOC
H
H
lycopalhine A (3)
dendrine (4)
dendrobine (1)
kopsanone (2)
N
COOMe
HO
H
N
H
H
H
Me
H
HO
N
R1
N
H
Key words dendrobine, [5/5/6] aza-tricyclic, metathesis, allylation,
NOE, BF3·OEt2
H
OH
R2
H
kopsinidines
mubironine C (5)
[5/5/6] aza-tricyclic core
R1 = R2 = H, 6
R1 = H, R2 = OMe, 7
R1 = R2 = OCH2O, 8
The total synthesis of alkaloids has been considered as a
challenging task in organic synthesis.1a Alkaloids such as
dendrobine (1), kopsanone (2), and lycopalhine A (3) con-
tain the [5/5/6] aza-tricyclic core as a common structural
element. Dendrobine (1), a tetracyclic pyrrolidine alkaloid
isolated from the Dendrobium nobile plant, shows analgesic
and antipyretic activity.1 Dendrine (4) and mubironine C
(5) are structurally related alkaloids to dendrobine (1), orig-
inated from similar orchid species.2 Kopsinidines 6–8 and
kopsanone (2) are monoterpenoid alkaloids isolated from
Kopsia officinalis plant that exhibits anti-inflammatory, an-
tirheumatic, and cholinergic effects.3 Interestingly, lycopal-
hine A (3) is a hexacyclic lycopodium alkaloid isolated from
Palhinhaea cernua plant, a family of lycopodiaceae.4 Total
synthesis of these pyrrolidine-based alkaloids have gained
considerable interest in recent years due to their unique
structural features and a wide range of biological activities
(Figure 1).
Figure 1 Alkaloids containing [5/5/6] aza-tricyclic core
have disclosed the [5/5/6] aza-tricyclic Kende intermediate
towards asymmetric synthesis of dendrobine in seven steps
in 15% overall yield (Scheme 1a).5 In 2018, Williams and
Trauner have reported the synthesis of 5-deoxymubironine
C in eight steps in 7% overall yield (Scheme 1b).6
Here, we have developed a simple and stereoselective
metathesis strategy to synthesize [5/5/6] pyrrolidine aza-
tricyclic core, which produced more than 50% overall yield
in a stereoselective manner (Scheme 1c). Further, we ex-
panded this strategy to assemble [5/5/8] aza-tricyclic core
successfully. The methodology reported to pyrrolidine cores
may be useful in generating compounds suitable for materi-
al science and bioactive targets.
Synthesis of tricyclic [5/5/6] pyrrolidine unit is not a
trivial task. Previously, this pyrrolidine-based [5/5/6] tricy-
clic core has been assembled by a lengthy synthetic se-
quence in moderate yields. Recently, Chen and co-workers
Our synthesis starts with the preparation of known
endo-Diels–Alder (DA) adducts 9a–d,7 which on reduction
with NaBH4-I2 system8 at room temperature in CH2Cl2–
MeOH gave the hydroxyl derivatives 10a–d in good yields
© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, A–H
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
S. Kotha, S. Pulletikurti
Paper
Synthesis
a)
H
H
H
2 steps
TMS
H
Pd(OAc)2, PPh3
4 steps
Me
Br
N
NTs then RhCl(PPh3)
AgSbF6
N
Me
Br
Ts
H
O
MgBr
i-Pr
Kende intermediate
15% overall yield
i-Pr
O
H
b)
N
COOH
MeOOC
H
7 steps
OHC
o-DCB, 160 °C
N
Me
H
COOMe
5-deoxymubironine C
7% overall yield
(R)-carvone
c) this work
O
H
H
H
O
3 steps
RCM
H
O
NPh
NPh
H
H
N
Ph
O
[5/5/6] aza-tricyclic core
>50% overall yield
Scheme 1 (a) Earlier reported asymmetric pathway to dendrobine; (b) Previously reported azomethine ylide pathway to 5-deoxymubironine C;
(c) Proposed metathetic approach to [5/5/6] aza-tricyclic core
as a pure diastereomer (Scheme 2). We found that electron-
withdrawing groups (CN, Br) at para-position produce ex-
cellent yields of 10b and 10c, whereas electron-donating
group (Me) gave moderate yields. The stereochemistry of
10 was confirmed by NOE experiment. Recently, Bergens
and co-workers have reported an enantioselective catalytic
hydrogenation of amides and imides through base-cata-
lyzed bifunctional addition. They assigned the stereochem-
istry of the hydroxyl group with the aid of single crystal X-
ray analysis data of the corresponding carbamate deriva-
tive.9 Further, addition of allylsilane via N-acyliminium ions
is one of the mostly used methodology to synthesize func-
tionalized lactams.10 Thus, allylation of 10 with allyl TMS
was accomplished in the presence of BF3·OEt2 at –78 °C to
deliver the allyl derivative 11 in good yield with a -selec-
tivity (Scheme 2).
The stereochemistry of allyl derivative 11 was derived
by attack of allyl TMS on acyliminium ion from less hin-
dered side as proposed in the mechanism (Scheme 3) and
the stereochemistry was confirmed by NOE study and fur-
ther supported by single-crystal X-ray diffraction studies.11
We have studied the allylation sequence at different
temperatures (–78 °C to rt) to improve the yield and found
that this reaction gave stereocontrolled product 11 even at
H
H
O
H
OH
TMS
NaBH4-I2
N
N
catalyst
temp/time
CH2Cl2–MeOH
N
H
H
H
11ca
rt, 8–12 h
R
O
O
O
R
11
R
10
9
10a, R = H, 81%
10b, R = CN, 93%
10c, R = Br, 89%
10d, R = Me, 57%
11db
Scheme 2 Synthesis of -allyl precursor 11. a Crystal XRD of 11c with thermal ellipsoids drawn at 50% probability level. b Crystal XRD of 11d with
thermal ellipsoids drawn at 50% probability level.
© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, A–H

C
S. Kotha, S. Pulletikurti
Paper
Synthesis
H-OH
SiMe3
H
OH
H
H
SiMe3
NAr
NAr
NAr
H
NAr
H
O
H
O
O
O
SiMe3
10
acyliminium ion
11
Scheme 3 Possible mechanism for allylation
0 °C and also at room temperature in excellent yields (Table
1).
metathesis (ROM) products 12b and 12c. Later, these were
again subjected to the ring-closing metathesis (RCM) using
G-II catalyst at room temperature to deliver the desired aza-
tricyclic derivatives 13b and 13c in 84% and 76% yield, re-
spectively (Scheme 4).12
Table 1 Reaction Conditions of Allylation of 10
Entry Compound R
Catalyst
Temp (°C) Time (h) Conversion (%)a
Table 2 Reaction Optimization of ROM of 11b
1
2
10b
10d
10c
10c
10c
10c
10c
10c
10c
10c
CN
Me
Br
Br
Br
Br
Br
Br
Br
Br
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
BF3·OEt2
TiCl4
–78
–78
–78
–40
–20
0
15
15
15
14
15
8
73
47
65
78
97
98
100
40
45
0
Entry
Catalyst (mol%)
Solvent/temp/time
Yield of 12b (%)a
3
1
2
3
4
5
6
7
G-I (5 mol%)
G-I (10 mol%)
G-I (10 mol%)
G-II (5 mol%)
GH-I (5 mol%)
GH-I (5 mol%)
GH-II (5 mol%)
CH2Cl2/rt/12 h
CH2Cl2/rt/12 h
toluene/reflux/8 h
CH2Cl2/rt/12 h
CH2Cl2/rt/12 h
toluene/reflux/8 h
CH2Cl2/rt/12 h
34
26
28
25
72
70
36
4
5
6
7
rt
6
8
0
8
9
CF3CO2H
SiCl4
0
8
10
0
8
a Isolated yield.
a Percentage of conversion of allylation is based on the 1H NMR data.
Along similar lines, the hydroxy derivative 10b was
treated with allyl bromide in the presence of NaH to furnish
the O-allyl derivative 14 in 98% yield.13 Further, this O-allyl
derivative 14 was subjected to ROM to yield the compound
15, which on treatment with G-I and G-II catalysts did not
produce the expected ring-closure product 16 (Scheme 5).
Further, -allyl derivatives 11b and 11c were subjected
to metathesis using Grubbs 1st, 2nd generation and Hovey-
da–Grubbs 1st and 2nd catalysts under different condi-
tions.12 Unfortunately, we did not observe the ring-rear-
rangement metathesis (RRM) product 13 under these
conditions (Table 2); however, we found the ring-opening
O
O
ethylene
H
Br
NaH, THF
H
H
O
G-II, CH2Cl2
NAr
GH-I, CH2Cl2
10b
NAr
ethylene
ethylene
H
H
rt, 8 h, 89%a
H
H
0 °C, 3 h
98%
NAr
G-II (5 mol%)
catalyst, temp
H
H
15
H
O
NAr
O
16
NAr
O
NAr
solvent, time
14
CH2Cl2, rt
5 h
H
H
O
O
Ar =
CN
O
11b,c
12b,c
12c = 63%
13b = 84%
13c = 76%
Scheme 5 Synthesis of the compound 15 (70% of conversion from 14
to 15 by 1H NMR analysis). a Yield is based on the recovery of 30% start-
ing material.
Ar =
NOE Study
We have performed the NOE studies of compounds 10,
11, 12, and 14 to establish the relative stereochemistry of
alkyl and vinyl side chains. For example, Hd proton of cyclic
CHd=CH moiety shows strong NOE correlation with Ha of
Br
CN
b
c
Scheme 4 Synthesis of aza-tricyclic core 13 using metathesis strategy
© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, A–H

D
S. Kotha, S. Pulletikurti
Paper
Synthesis
HO
B
ethylene
H
H
H
GH-I, CH2Cl2
G-II (5 mol%)
OH
NAr
NAr
10c
NAr
rt, 15 h, 91%a
CH2Cl2, rt
5 h, 62%a
BF3·OEt2
H
H
H
O
O
O
–78 °C to rt, 12 h
58%
18
17
19
Br
Ar =
Scheme 6 Synthesis of compound 19 (65% of conversion from 17 to 18 by 1H NMR analysis). a Yields are based on recovery of 35% starting material.
CHa–NPh of compounds 10, 11, and 14. Additionally, meth-
ylene protons (Hj, Hk) of CH2=CH exhibit NOE correlation
with the Hg of bridged CH2 in compound 14. These results
indicated that the stereochemistry of hydroxyl group of 10
is assigned as -orientation. Similarly, methylene protons
(Hh and Hi) of allyl group exhibit strong NOE with
the bridge protons (Hb and Hc) in compound 11 and Ha of
CHa–NPh of the compound 12 shows NOE correlation with
Hd of vinylic CHd=CH2 group of 12. These observations sup-
port the stereochemistry of allyl group of 11 as  (Figure 2).
All commercially available reagents were used without further purifi-
cation and the reactions involving air-sensitive catalysts or reagents
were performed in degassed solvents. Moisture-sensitive materials
were transferred by using syringe-septum technique and the reac-
tions were maintained under N2 atmosphere. Analytical TLC was per-
formed on glass plates (7.5 × 2.5 cm) coated with Acme’s silica gel GF
254 (containing 13% CaSO4 as a binder) by using a suitable mixture of
EtOAc and PE for development. Column chromatography was per-
formed by using Acme’s silica gel (100–200 mesh) with an appropri-
ate mixture of EtOAc and PE. The coupling constants (J) are given in
hertz (Hz) and chemical shifts are denoted in parts per million (ppm)
downfield from internal standard TMS. Standard abbreviations are
used to denote spin multiplicities. IR spectra were recorded on Nico-
let Impact-400 FT-IR spectrometer. NMR spectra were generally re-
corded on a Bruker (AvanceTM 400 or AvanceTM III 500) spectrome-
Hg
Hf
Hh
Hi
Hb
Ha
Hb
Ha
Hd
He
OH
Hd
He
NAr
ter operating at 400 or 500 MHz for 1H and 100.6 or 125.7 MHz for 13
C
Hc
NAr
Hc
nuclei. The high-resolution mass spectrometric (HRMS) measure-
ments were carried out using a Bruker (Maxis Impact) or Micromass
Q-ToF spectrometer.
O
O
10
11
Hk
endo-Imides 9a–d;7 General Procedure
Hj
Hd
Hi
Ha
Hg
Hb
The known endo-imides 9a–d were prepared following the literature
Hb
Hf
procedure.7
Hh
NAr
Ha
O
Hd
He
Compound 9a
Hc
NAr
Hc
O
Off-white solid; mp 143.9–144.9 °C18a; Rf = 0.35 (20% EtOAc/hexane).
O
12
14
1H and 13C NMR spectra of compound 9a matched with the literature
reported values.7
Figure 2 NOE correlation of the compounds 10, 11, 12, and 14
Compound 9b
We have expanded this methodology to synthesize
[5/5/8] tricyclic derivative 19, which is difficult to assemble
by conventional methods. Addition of unsaturated boronic
acid such as pentenylboronic acid to N-acyliminium ions in
the presence of Lewis acid, for example, BF3·OEt2, copper
triflate, and Ca(II) catalysts, is not reported.14–17
White solid; mp 169–172 °C18b; Rf = 0.39 (20% EtOAc/hexane).
1H and 13C NMR spectra of compound 9b matched with the literature
reported values.7
Compound 9c
White solid; mp 153.4–154.6 °C18a; Rf = 0.38 (20% EtOAc/hexane).
Thus, the hydroxy derivative 10 was treated with pen-
tenylboronic acid in the presence of BF3·OEt2 at –78 °C to
deliver the pentenyl derivative 17 in 58% yield. This deriva-
tive 17 was further subjected to ROM using GH-I catalyst
followed by RCM using G-II catalyst to obtain the corre-
sponding aza-tricyclic analogue 19 in good yield (Scheme 6).12
1H and 13C NMR spectra of compound 9c matched with the literature
reported values.7
Compound 9d
Off-white solid; mp 158.2–158.6 °C18a; Rf = 0.36 (20% EtOAc/hexane).
1H and 13C NMR spectra of compound 9d matched with the literature
reported values.7
© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, A–H

E
S. Kotha, S. Pulletikurti
Paper
Synthesis
-Hydroxyl Lactams 10; General Procedure
Compound 10d
The respective imide 9 (1 mmol, 1 equiv) was dissolved in CH2Cl2–
MeOH (1:1, 20 mL) and I2 (catalytic amount) was added at rt under N2
atmosphere. Later, the resultant solution was stirred for 15 min at rt;
NaBH4 (5 mmol, 5 equiv) was added and the mixture was allowed to
stir for 8–12 h at rt. After completion of the reaction, solvents were
removed under reduced pressure. The residue was diluted with CH2-
Cl2, washed with H2O (2 × 30 mL), and concentrated to obtain the de-
sired compound as a pure diastereomer.
White solid; yield: 455 mg (57%) starting from 800 mg of 9d; mp
104.8–106.5 °C; Rf = 0.30 (30% EtOAc/hexane).
IR (neat): 2933, 1684, 1515, 1405, 1321, 842, 761 cm–1
.
1H NMR (CDCl3, 400 MHz):  = 7.23 (d, J = 8.40 Hz, 2 H), 7.14 (d, J =
8.24 Hz, 2 H), 6.22 (dd, J = 5.52, 2.78 Hz, 1 H), 6.13 (dd, J = 5.56, 2.78
Hz, 1 H), 4.88 (s, 1 H), 3.37–3.17 (m, 4 H), 2.71 (dd, J = 8.40, 4.14 Hz, 1
H), 2.32 (s, 3 H), 1.59 (d, J = 8.48 Hz, 1 H), 1.40 (d, J = 8.48 Hz, 1 H).
13C NMR (CDCl3, 100 MHz):  = 175.1 (C), 136.7 (C), 136.6 (CH), 134.5
(C), 133.4 (CH), 129.8 (CH), 124.7 (CH), 87.3 (CH), 51.4 (CH2), 49.5
(CH), 46.5 (CH), 45.8 (CH), 45.2 (CH), 21.1 (CH).
Compound 10a
Pale yellow liquid yield: 820 mg (81%) starting from 1.0 g of 9a;
Rf = 0.32 (30% EtOAc/hexane).
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C16H17NO2Na: 278.1153;
found: 278.1151.
IR (neat): 2946, 2931, 1687, 1551, 1392, 822, 771 cm–1
.
1H NMR (CDCl3, 500 MHz):  = 7.45–7.33 (m, 4 H), 7.26–7.19 (m, 1 H),
6.25 (dd, J = 5.40, 2.60 Hz, 1 H), 6.16 (dd, J = 5.45, 2.55 Hz, 1 H), 4.97
(d, J = 7.20 Hz, 1 H), 3.38–3.31 (m, 2 H), 3.26 (br s, 1 H), 2.88 (d,
J = 7.35 Hz, 1 H), 2.75 (dd, J = 8.25, 4.25 Hz, 1 H), 1.62 (dd, J = 8.55,
1.30 Hz, 1 H), 1.44 (dd, J = 8.50 Hz, 1 H).
13C NMR (CDCl3, 125 MHz):  = 175.2 (C), 137.1 (C), 136.7 (CH), 133.4
(CH), 129.3 (CH), 126.8 (CH), 124.6 (CH), 87.1 (CH), 51.4 (CH2), 49.6
(CH), 46.5 (CH), 45.9 (CH), 45.30 (CH).
-Allyl Lactams 11b–d and Lactam 17; General Procedure
BF3·OEt2 (4 mmol for 1 mmol of 10, 4 equiv) was added to a solution
of the respective -hydroxyl lactam derivative 10b–d (1 mmol, 1
equiv) at the given temperature (Table 1) and the mixture was stirred
for 15 min under N2 atmosphere. Next, allyl TMS or pentenylboronic
acid (4 mmol for 1 mmol of 10, 4 equiv) was added to the solution and
allowed to stir for 1 h at the same temperature. The mixture was
brought to rt over 4–8 h and the stirring was continued for 2 h at rt.
After completion of reaction, the mixture was quenched and washed
with H2O (2 × 25 mL). The organic layer was dried (Na2SO4) and con-
centrated under reduced pressure to obtain the desired compound as
a pure isomer. The crude product was purified by column chromatog-
raphy.
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C15H15NO2Na: 264.0996;
found: 264.0993.
Compound 10b
White solid; yield: 744 mg (93%) starting from 800 mg of 9b; mp
161.1–165.7 °C; Rf = 0.30 (30% EtOAc/hexane).
Compound 11b
IR (neat): 2925, 2228, 1683, 1510, 1392, 1304, 842, 760 cm–1
.
White solid; yield: 140 mg (73%) starting from 250 mg of 10b (yield
based on 30% recovered starting material); mp 105.8–108.2 °C;
Rf = 0.55 (20% EtOAc/hexane).
IR (neat): 2935, 2222, 1692, 1508, 1385, 1295, 915, 763 cm–1.
1H NMR (CDCl3, 400 MHz):  = 7.62 (d, J = 8.56 Hz, 2 H), 7.55–7.51 (m,
2 H), 6.19 (s, 2 H), 5.74–5.62 (m, 1 H), 5.18–5.02 (m, 2 H), 3.79 (dt, J =
7.72, 5.30 Hz, 1 H), 3.34 (br s, 1 H), 3.29 (dd, J = 9.16, 9.14 Hz, 1 H),
3.14 (br s, 1 H), 2.65 (ddd, J = 9.16, 2.52 Hz, 1 H), 2.38–2.30 (m, 1 H),
2.25–2.16 (m, 1 H), 1.61 (d, J = 8.50 Hz, 1 H), 1.43 (d, J = 8.50 Hz, 1 H).
13C NMR (CDCl3, 100 MHz):  = 175.1 (C), 141.7 (C), 137.6 (CH), 133.6
(CH), 133.1 (CH), 132.2 (CH), 123.0 (CH), 119.6 (CH2), 118.8 (C), 108.3
(C), 61.1 (CH), 51.1 (CH2), 51.0 (CH), 46.8 (CH), 46.0 (CH), 40.7 (CH),
38.2 (CH2).
1H NMR (CDCl3, 400 MHz):  = 7.70 (d, J = 8.76 Hz, 2 H), 7.60 (d,
J = 8.72 Hz, 2 H), 6.17 (dd, J = 5.64, 2.80 Hz, 1 H), 6.09 (dd, J = 5.60,
2.92 Hz, 2 H), 5.00 (s, 1 H), 3.38–3.30 (m, 2 H), 3.28 (br s, 1 H), 2.76
(dd, J = 8.28, 4.24 Hz, 1 H), 1.62 (d, J = 8.60 Hz, 1 H), 1.42 (d, J = 8.68
Hz, 1 H).
13C NMR (CDCl3, 100 MHz):  = 175.5 (C), 141.5 (C), 136.5 (CH), 133.5
(CH), 133.1 (CH), 122.4 (CH), 118.7 (C), 108.8 (C), 86.5 (CH), 51.4
(CH2), 49.9 (CH), 46.7 (CH), 46.4 (CH), 45.4 (CH).
HRMS (ESI, Q-ToF): m/z [M
+
Na]+ calcd for C16H14N2O2Na:
289.0947; found: 289.0946.
Compound 10c
White solid; yield: 890 mg (89%) starting from 1.0 g of 9c; mp 186.1–
188.3 °C; Rf = 0.29 (30% EtOAc/hexane).
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C19H18N2ONa: 313.1312;
found: 313.1311.
IR (neat): 2922, 1689, 1516, 1429, 989, 770 cm–1
.
1H NMR (CDCl3, 400 MHz):  = 7.43 (dt, J = 8.84, 2.04 Hz, 2 H), 7.30 (dt,
J = 8.88, 2.04 Hz, 2 H), 6.17 (dd, J = 5.64, 2.96 Hz, 1 H), 6.09 (dd, J =
5.56, 2.88 Hz, 1 H), 4.85 (d, J = 6.32 Hz, 1 H), 3.61 (d, J = 8.08 Hz, 1 H),
3.31 (br t, J = 1.26 Hz, 1 H), 3.27–3.20 (m, 2 H), 2.70 (ddd, J = 8.52,
4.24, 0.76 Hz, 1 H), 1.60 (dt, J = 8.55, 1.52 Hz, 1 H), 1.38 (d, J = 8.52 Hz,
1 H).
13C NMR (CDCl3, 100 MHz):  = 175.7 (C), 136.4 (CH), 133.5 (CH),
132.1 (CH), 125.4 (CH), 119.7 (C), 87.2 (CH), 51.4 (CH2), 49.6 (CH),
46.6 (CH), 46.1 (CH), 45.3 (CH).
Compound 11c
White solid; yields starting from 300 mg (0.874 mmol) of 10c are
mentioned in Table 1; mp 106.1–110.6 °C; Rf = 0.57 (20% EtOAc/hex-
ane).
IR (neat): 2930, 1689, 1491, 1387, 1290, 826 cm–1
1H NMR (CDCl3, 400 MHz):  = 7.44 (d, J = 8.76 Hz, 2 H), 7.17 (d,
J = 8.76 Hz, 2 H), 6.21 (s, 2 H), 5.74–5.60 (m, 1 H), 5.12 (d, J = 10.12 Hz,
1 H), 5.06 (dd, J = 17.08, 1.38 Hz, 1 H), 3.68–3.61 (m, 1 H), 3.31 (s, 1 H),
3.23 (dd, J = 9.24, 9.14 Hz, 1 H), 3.10 (d, J = 3.10 Hz, 1 H), 2.61 (qt,
J = 10.56, 2.22 Hz, 1 H), 2.33–2.24 (m, 1 H), 2.20–2.10 (m, 1 H), 1.59
(d, J = 8.38 Hz, 1 H), 1.41 (d, J = 8.44 Hz, 1 H).
.
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C15H14BrNO2Na: 342.0099;
found: 342.0100.
© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, A–H

F
S. Kotha, S. Pulletikurti
Paper
Synthesis
13C NMR (CDCl3, 100 MHz):  = 174.6 (C), 137.5 (CH), 136.6 (C), 133.6
(CH), 132.5 (CH), 132.1 (CH), 125.8 (CH), 119.2 (CH2), 61.9 (CH), 51.0
(CH2), 50.6 (CH), 46.6 (CH), 45.6 (CH), 40.8 (CH), 38.3 (CH2).
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C18H18BrNONa: 366.0464;
found: 366.0461.
13C NMR (CDCl3, 125 MHz):  = 175.6 (C), 141.7 (C), 136.5 (CH), 133.3
(CH), 133.2 (CH), 132.8 (CH), 122.4 (CH), 118.6 (C), 117.8 (CH2), 108.6
(C), 91.4 (CH), 66.2 (CH2), 51.4 (CH2), 49.9 (CH), 46.2 (CH), 45.5 (CH),
42.8 (CH).
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C19H18N2O2Na: 329.1259;
found: 329.1260.
Compound 11d
Ring-Opening Metathesis; General Procedure
Colorless liquid; yield: 107 mg (47%) starting from 300 mg of 10d
(yield based on 30% recovered starting material); Rf = 0.55 (20% EtO-
Ac/hexane).
The respective compound 11, 17, or 14 was dissolved in an anhyd sol-
vent (7 mM, CH2Cl2, or toluene) and degassed with N2 followed by
ethylene for about 20 min. To this, was added Grubbs catalyst (5 mol%
or 10 mol%) and stirred (as described in Table 2 conditions) under
ethylene atmosphere. Solvent was removed and the crude product
was purified by column chromatography to obtain the desired product.
IR (neat): 2927, 1688, 1395, 915, 816 cm–1
.
1H NMR (CDCl3, 400 MHz):  = 7.17–7.10 (m, 4 H), 6.28–6.21 (m, 2 H),
5.77–5.65 (m, 1 H), 5.16–5.04 (m, 2H), 3.60 (dt, J = 7.88, 2.74 Hz, 1 H),
3.36–3.30 (m, 1 H), 3.23 (dd, J = 9.33, 9.16 Hz, 1 H), 3.12–3.07 (m, 1 H),
2.61 (qd, J = 10.56, 2.24 Hz, 1 H), 2.31 (s, 3 H), 2.30–2.25 (m, 1 H),
2.21–2.05 (m, 1 H), 1.60 (dt, J = 8.44, 1.56 Hz, 1 H), 1.42 (d, J = 8.44 Hz,
1 H).
13C NMR (CDCl3, 100 MHz):  = 174.7 (C), 137.5 (CH), 136.1 (C), 134.9
(C), 133.6 (CH), 133.0 (CH), 129.7 (CH), 124.8 (CH), 118.8 (CH2), 62.4
(CH), 51.0 (CH2), 50.5 (CH), 46.7 (CH), 45.6 (CH), 40.9 (CH), 38.5 (CH2),
21.1 (CH).
Compound 12b
Colorless liquid; yields starting from 70 mg of 11b (0.22 mmol) are
given in Table 2; Rf = 0.6 (20% EtOAc/hexane).
IR (neat): 2925, 2227, 1703, 1509, 1386, 1295, 920, 760 cm–1
1H NMR (CDCl3, 400 MHz):  = 7.63 (s, 4 H), 6.10–5.98 (m, 1 H), 5.94–
5.82 (m, 1 H), 5.63–5.49 (m, 1 H), 5.26–4.93 (m, 6 H), 4.24 (pent,
J= 3.04 Hz, 1 H), 3.22 (t, J = 8.68 Hz, 1 H), 2.96–2.82 (m, 2 H), 2.75 (td,
J = 11.20, 2.64 Hz, 1 H), 2.34–2.15 (m, 2 H), 1.96–1.87 (m, 1 H), 1.45 (q,
J = 12.45 Hz, 1 H).
.
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C19H21NONa: 302.1515;
found: 302.1515.
13C NMR (CDCl3, 100 MHz):  = 173.7 (C), 141.7 (C), 137.5 (CH), 133.1
(CH), 131.7 (CH), 122.8 (CH), 120.0 (CH2), 117.1 (CH2), 115.1 (CH2),
108.3 (C) 58.5 (CH), 51.6 (CH), 47.3 (CH), 46.4 (CH), 42.7 (CH), 37.7
(CH2), 35.2 (CH2).
HRMS (ESI, Q-ToF): m/z [M + H]+ calcd for C21H22N2O: 319.1804;
found: 319.1805.
Compound 17
Colorless liquid; yield: 100 mg (58%) starting from 150 mg of 10c;
Rf = 0.50 (20% EtOAc/hexane).
IR (neat): 2926, 2857, 1691, 1484, 1384, 914 cm–1
.
1H NMR (CDCl3, 400 MHz):  = 7.44 (d, J = 8.16 Hz, 2 H), 7.31 (d, J = 8.3
Hz, 2 H), 6.21 (s, 1 H), 6.12 (s, 1 H), 5.80–5.66 (m, 1 H), 5.03–4.90 (m, 2
H), 4.72 (s, 1 H), 3.43–3.27 (m, 4 H), 3.19 (s, 1 H), 2.76 (br s, 1 H), 2.06
(q, J = 6.90 Hz, 1 H), 1.70–1.53 (m, 4 H), 1.46 (d, J = 8.60 Hz, 1 H).
13C NMR (CDCl3, 100 MHz):  = 175.4 (C), 137.9 (CH), 136.9 (C), 136.8
(CH), 136.7 (CH), 133.2 (CH), 132.1 (CH), 125.6 (C), 125.2 (CH), 119.5
(C), 115.3 (CH2), 92.7 (CH), 64.7 (CH2), 51.5 (CH2), 49.8 (CH), 46.1 (CH),
45.6 (CH), 43.3 (CH), 30.3 (CH2), 28.9 (CH2).
Compound 12c
Colorless liquid; yield: 48 mg (63%) from 70 mg of 11c (0.19 mmol);
Rf = 0.6 (20% EtOAc/hexane).
IR (neat): 2923, 1702, 1641, 1490, 1291, 916, 757 cm–1
.
1H NMR (CDCl3, 400 MHz):  = 7.47 (dt, J = 8.84, 2.04 Hz, 2 H), 7.31 (dt,
J = 8.78, 2.07 Hz, 2 H), 6.13–6.04 (m, 1 H), 5.97–5.86 (m, 1 H), 5.64–
5.51 (m, 1 H), 5.65–5.51 (m, 6 H), 4.14 (dt, J = 6.52, 3.24 Hz, 1 H), 3.19
(t, J = 8.74 Hz, 1 H), 2.94–2.83 (m, 2 H), 2.74 (td, J = 8.48, 3.16 Hz, 1 H),
2.30–2.13 (m, 2 H), 1.92 (dt, J = 12.36, 5.89 Hz, 1 H), 1.50 (q, J = 12.45
Hz, 1 H).
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C21H22N2ONa: 341.1625;
found: 341.1624.
Compound 14
NaH (60% dispersed in paraffin, 115 mg, 4.7 mmol, 5 equiv) was
washed with anhyd PE (2 × 20 mL) and dried under N2 before being
suspended in anhyd THF (20 mL). To this suspension, was added com-
pound 10b (300 mg, 0.94 mmol) and stirred for 10 min at rt. After the
reaction mixture was cooled to 0 °C, allyl bromide (0.45 mL, 2.82
mmol, 3 equiv) was added and allowed to stir for overnight at rt. After
the completion of reaction, THF was removed and the residue was
suspended in EtOAc (30 mL). The suspension was washed with H2O
(2 × 20 mL), the organic layer was separated, and dried (Na2SO4). Sol-
vents were removed under reduced pressure to obtain the compound
14 as a pure product; yield: 330 mg (98%); colorless liquid; Rf = 0.35
(30% EtOAc/hexane).
13C NMR (CDCl3, 100 MHz):  = 173.2 (C), 137.9 (CH), 137.7 (CH),
136.6 (C), 132.2 (CH), 132.1 (CH), 125.4 (CH), 119.6 (CH2), 118.9 (C),
116.8 (CH), 114.8 (CH), 59.1 (CH), 51.4 (CH), 47.2 (CH), 46.4 (CH), 42.9
(CH), 37.7 (CH2), 35.3 (CH2).
HRMS (ESI, Q-ToF): m/z [M + H]+ calcd for C20H22BrNO: 394.0777;
found: 394.0775.
Compound 18
Colorless liquid; yield: 45 mg (91%) from 70 mg of 17 (0.22 mmol)
(yield based on 35% recovered starting material); Rf = 0.6 (20%
EtOAc/hexane).
IR (neat): 2972, 2223, 1708, 1508, 1385, 1064, 841, 748 cm–1
.
IR (neat): 2937, 1691, 1491, 1385, 1292, 827, 757 cm–1
.
1H NMR (CDCl3, 400 MHz):  = 7.65–7.60 (m, 2 H), 7.59–7.54 (m, 2 H),
6.15 (s, 1 H), 6.06 (d, J = 2.89 Hz, 1 H), 5.88–5.76 (m, 1 H), 5.23 (d, J =
17.16 Hz, 1 H), 5.15 (d, J = 10.40 Hz, 1 H), 4.87 (s, 1 H), 3.95 (d, J = 5.52
Hz, 2 H), 3.37–3.30 (m, 2 H), 3.18 (s, 1 H), 2.84–2.77 (m, 1 H), 1.52 (dd,
J = 61.50, 8.48 Hz, 2 H).
1H NMR (CDCl3, 400 MHz):  = 7.49–7.44 (m, 2 H), 7.40–7.36 (m, 2 H),
6.19–6.10 (m, 1 H), 5.97–5.89 (m, 1 H), 5.75–5.66 (m, 1 H), 5.24–5.04
(m, 6 H), 3.34–3.23 (m, 3 H), 2.94–2.84 (m, 2 H), 2.80 (t, J = 8.60 Hz, 1
H), 2.00 (q, J = 7.15 Hz, 2 H), 1.95–1.87 (m, 1 H), 1.70–1.64 (m, 1 H),
1.59–1.53 (m, 2 H), 1.39 (q, J = 12.52 Hz, 1 H).
© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, A–H

G
S. Kotha, S. Pulletikurti
Paper
Synthesis
13C NMR (CDCl3, 100 MHz):  = 173.5 (C), 137.9 (CH), 137.5 (CH),
137.3 (CH), 136.8 (C), 132.3 (CH), 132.1 (CH), 125.3 (CH), 125.1 (CH),
119.4 (C), 117.1 (CH2), 115.2 (CH2), 115.1 (CH2), 91.1 (CH), 65.3 (CH2),
50.5 (CH), 47.0 (CH), 45.8 (CH), 45.2 (CH), 35.0 (CH2), 30.2 (CH2), 28.8
(CH2).
13C NMR (CDCl3, 100 MHz):  = 176.9 (C), 139.3 (CH), 136.8 (C), 132.1
(CH), 131.5 (CH), 126.1 (CH), 125.7 (CH), 114.9 (CH2), 57.8 (CH), 51.6
(CH), 50.7 (CH), 44.9 (CH), 40.1 (CH), 38.3 (CH), 30.2 (CH2).
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C18H18BrNONa: 366.0461;
found: 366.0464.
HRMS (ESI, Q-ToF): m/z [M
+
Na]+ calcd for C23H26N2ONa:
369.1935; found: 369.1937.
Compound 19
Colorless liquid; yield: 13 mg (62%) starting from 25 mg of 18;
Rf = 0.46 (20% EtOAc/hexane).
Compound 15
Colorless liquid; yield: 68 mg (89%) from 100 mg of 14 (yield based on
30% recovered starting material); Rf = 0.45 (30% EtOAc/hexane).
IR (neat): 2923, 2857, 1710, 1491, 1384, 1286, 1074, 915 cm–1
.
1H NMR (CDCl3, 500 MHz):  = 7.53–7.47 (m, 2 H), 7.41–7.34 (m, 2 H),
5.26–5.19 (m, 1 H), 5.18–5.06 (m, 2 H), 4.39–4.09 (m, 1 H), 3.84–3.37
(m, 2 H), 3.26–3.24 (m, 1 H), 3.01–2.84 (m, 2 H), 2.77 (t, J = 8.91 Hz, 1
H), 2.08–1.89 (m, 2 H), 1.54–1.28 (m, 6 H).
13C NMR (CDCl3, 100 MHz):  = 173.1, 137.3, 136.9, 136.0, 132.3,
125.3, 125.2, 119.7, 118.1, 117.1, 115.3, 114.2, 85.1, 50.2, 50.0, 49.4,
46.7, 45.1, 34.9.
IR (neat): 2863, 2227, 1713, 1499, 1400, 1058, 932, 761 cm–1
.
1H NMR (CDCl3, 400 MHz):  = 7.74–7.69 (m, 2 H), 7.66–7.60 (m, 2 H),
6.20–6.09 (m, 1 H), 5.96–5.86 (m, 1 H), 5.84–5.73 (m, 1 H), 5.36 (d, J =
1.00 Hz, 1 H), 5.25–5.07 (m, 6 H), 3.90 (dt, J = 5.52, 1.32 Hz, 2 H), 3.31
(t, J = 8.10 Hz, 1 H), 3.00–2.84 (m, 3 H), 1.98–1.86 (m, 1 H), 1.37 (q, J =
12.46 Hz, 1 H).
13C NMR (CDCl3, 100 MHz):  = 173.8, 141.7, 137.0, 133.3, 133.0,
122.5, 118.7, 117.9, 117.5, 115.4, 108.8, 90.0, 66.7, 50.5, 47.1, 45.1,
34.9, 29.8.
HRMS (ESI, Q-ToF): m/z [M + H]+ calcd for C20H22BrNO: 394.0775;
found: 394.0776.
HRMS (ESI, Q-ToF): m/z [M + H]+ calcd for C21H22N2O2: 357.4083;
found: 357.4085.
Funding Information
We are thankful to the Science and Engineering Research Board
(EMR/2015/002053), New Delhi and CSIR [02(0272)/16/EMR-II], New
Delhi for financial support. SP thanks UGC, New Delhi for financial
Ring-Closing Metathesis; General Procedure
The respective compound 12 or 18 was dissolved in an anhyd solvent
(7 mM, CH2Cl2 or toluene) and degassed with N2 followed by ethylene
for about 20 min. To this, Grubbs 2nd generation catalyst (G-II, 5
mol%) was added and stirred for 5 h at rt under ethylene atmosphere.
Solvents were removed and purified by column chromatography to
obtain the desired product.
support and the award of a SRF.
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Acknowledgment
SK thanks the DST for the award of a J. C. Bose fellowship (SR/S2/JCB-
33/2010). We thank Dr. Saidulu Todeti for the preparation of the com-
pound 9b. We also thank Ms. Saima Ansari and Mr. Darshan Mhatre,
Dept. of Chemistry, IIT Bombay for collecting the crystal data. SK is
thankful to Praj industries for supporting the award of Pramod
Chaudhari Chair Professor (Green Chemistry).
Compound 13b
Colorless liquid; yield: 30 mg (84%) starting from 40 mg of 12b;
Rf = 0.48 (20% EtOAc/hexane).
IR (neat): 2876, 2219, 1717, 1476, 990, 755 cm–1
1H NMR (CDCl3, 400 MHz):  = 7.69–7.63 (m, 2 H), 7.39–7.34 (m, 2 H),
6.01–5.93 (m, 1 H), 5.83–5.66 (m, 2 H), 5.08 (dt, J = 16.92, 1.30 Hz, 1
H), 4.97 (dt, J = 10.20, 2.48 Hz, 1 H), 3.87 (td, J = 10.44, 3.80 Hz, 1 H),
3.36–3.23 (m, 1 H), 3.15 (dd, J = 11.76, 6.92 Hz, 1 H), 2.86–2.75 (m, 1
H), 2.61–2.50 (m, 2 H), 2.44–2.34 (m, 1 H), 2.09–2.00 (m, 1 H), 1.80–
1.69 (m, 1 H).
.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0039-1690620.
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13C NMR (CDCl3, 100 MHz):  = 176.7 (C), 141.9 (C), 139.0 (CH), 132.9
(CH), 131.5 (CH), 125.9 (CH), 123.5 (CH), 118.9 (C), 115.2 (CH2), 108.5
(C), 57.3 (CH), 51.4 (CH), 50.7 (CH), 45.0 (CH), 40.2 (CH), 38.2 (CH2),
30.2 (CH2).
HRMS (ESI, Q-ToF): m/z [M + Na]+ calcd for C19H18N2ONa: 313.1316;
found: 313.1318.
References
(1) (a) Nawrat, C. C.; Moody, C. J. Angew. Chem. Int. Ed. 2014, 53,
2056. (b) Inubushi, Y.; Sasaki, Y.; Tsuda, Y.; Yasui, B.; Konita, T.;
Matsumoto, J.; Katarao, E.; Nakano, J. Tetrahedron 1964, 20,
2007. (c) Chen, K. K.; Chen, A. L. J. Biol. Chem. 1935, 111, 653.
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(b) Morita, H.; Fujiwara, M.; Yoshida, N.; Kobayashi, J. I. Tetrahe-
dron 2000, 56, 5801.
Compound 13c
Colorless liquid; yield: 28 mg (76%) starting from 40 mg of 12c;
Rf = 0.50 (20% EtOAc/hexane).
(3) (a) Zeng, T.; Wu, X.-Y.; Yang, S.-X.; Lai, W.-C.; Shi, S.-D.; Zou, Q.;
Liu, Y.; Li, L.-M. J. Nat. Prod. 2017, 80, 864. (b) Yap, W.-S.; Gan,
C.-Y.; Sim, K.-S.; Lim, S.-H.; Low, Y.-Y.; Kam, T.-S. J. Nat. Prod.
2016, 79, 230. (c) Leng, L.; Zhou, X.; Liao, Q.; Wang, F.; Song, H.;
Zhang, D.; Liu, X.-Y.; Qin, Y. Angew. Chem. Int. Ed. 2017, 56, 3703.
IR (neat): 2925, 2853, 1714, 1490, 992, 762 cm–1
.
1H NMR (CDCl3, 400 MHz):  = 7.49 (d, J = 8.68 Hz, 2 H), 7.11 (d,
J = 8.72 Hz, 2 H), 5.99–5.92 (m, 1 H), 5.85–5.73 (m, 2 H), 5.08 (dt,
J = 16.96, 1.25 Hz, 1 H), 4.99 (dt, J = 10.28, 1.16 Hz, 1 H), 3.81 (td,
J = 14.52, 3.38 Hz, 1 H), 3.33–3.23 (m, 1 H), 3.11 (dd, J = 11.88, 6.96 Hz,
1 H), 2.83–2.73 (m, 1 H), 2.55–2.48 (m, 1 H), 2.47–2.34 (m, 2 H), 2.08–
2.02 (m, 1 H), 1.79–1.72 (m, 1 H).
© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, A–H

H
S. Kotha, S. Pulletikurti
Paper
Synthesis
(4) (a) Dong, L.-B.; Yang, J.; He, J.; Luo, H.-R.; Wu, X.-D.; Deng, X.;
Peng, L.-Y.; Cheng, X.; Zhao, Q.-S. Chem. Commun. 2012, 48,
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2000, 609, 137. (b) Haldar, P.; Ray, J. K. Tetrahedron Lett. 2003,
44, 8229.
(9) John, J. M.; Takebayashi, S.; Dabral, N.; Miskolzie, M.; Bergens, S.
H. J. Am. Chem. Soc. 2013, 135, 8578.
(10) (a) Yazici, A.; Wille, U.; Pyne, S. G. J. Org. Chem. 2016, 81, 1434.
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2008, 73, 2935.
(11) (a) CCDC 1887403 (11c) and 1887402 (11d) contain the supple-
mentary crystallographic data for this paper. The data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/getstructures. (b) NOE
data of compound 11c and 11d are provided in the Supporting
Information.
(12) (a) Kotha, S.; Pulletikurti, S. RSC Adv. 2018, 8, 14906. (b) Ogba,
O. M.; Warner, N. C.; O’Leary, D. J.; Grubbs, R. H. Chem. Soc. Rev.
2018, 47, 4510. (c) Connon, S. J.; Blechert, S. Angew. Chem. Int.
Ed. 2003, 42, 1900. (d) Grela, K. Beilstein J. Org. Chem. 2015, 11,
1639. (e) Randall, M. L.; Snapper, M. L. J. Mol. Catal. A: Chem.
1998, 133, 29. (f) Acharyya, R. K.; Rej, R. K.; Nanda, S. J. Org.
Chem. 2018, 83, 2087. (g) Bose, S.; Ghosh, M.; Ghosh, S. J. Org.
Chem. 2012, 77, 6345. For previous work of our research group
in the field, see: (h) Kotha, S.; Rao, N. N.; Ravikumar, O.;
Sreevani, G. Tetrahedron Lett. 2017, 58, 1283. (i) Kotha, S.;
Chinnam, A. K.; Shirbhate, M. E. J. Org. Chem. 2015, 80, 9141.
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© 2019. Thieme. All rights reserved. — Synthesis 2019, 51, A–H